COLUMBIA, Mo. -- Every day, people come in contact with a toxic, metallic chemical known as cadmium through the air, water and food from contaminated crops. One crop that specifically accumulates cadmium is tobacco, which makes smokers susceptible to significantly higher levels of the metal in their bodies compared to non-smokers. However, non-smokers can be exposed to higher levels of cadmium via second-hand smoke. Now, a University of Missouri-Columbia researcher has found that low levels of cadmium in the brain may increase the negative effects that drugs, such as cocaine and methamphetamines, have on humans and may contribute to severe neurological disorders.

"The study shows that even low-level exposure to a 'heavy' metal like cadmium can cause a change in the functions of neurons in the brain and the behavioral response to drugs of abuse," said Dennis Miller, assistant professor of psychology at MU. "The implication is that toxicants may encourage drug users to increase their drug intake or increase the odds of developing some neurological disorders such as schizophrenia and Parkinson's disease."

Miller and Stan Casteel of the MU Veterinary Diagnostic Laboratory tested rats that received cadmium chloride for 30 days along with rats that received regular food. Miller said the level of cadmium being fed to the rats increased the levels of cadmium in the bloodstream to levels that approximated pack-a-day tobacco smokers. The rats were tested for their behavior and neurochemical responses to amphetamines, drugs that increase the activity of the dopamine, a neurotransmitter in the brain.

Miller found the brains of rats exposed to the cadmium diet were less sensitive to the amphetamine than those that received regular food. These results, Miller said, suggest that dietary cadmium exposure produces a subtle nuerotoxic effect on neurons in the brain. Miller believes this indicates that dietary cadmium exposure could diminish the reinforcing properties of amphetamines, which means drug users might need more of the drug to achieve a "high." This should be a cause for concern for tobacco and drug users, Miller said.

"Tobacco smoking produces a long-lasting increase in the body burden of cadmium, and therefore this population might administer higher doses of amphetamine than non-smokers to achieve comparable reinforcing effects," Miller said. "Self-administration of higher amphetamine doses would increase the risk of drug toxicity or mortality."

Another implication of the study, Miller said, is the possible role cadmium exposure might have in the development of neurological and psychological disorders.

"A number of behavioral and psychiatric disorders are associated with a change in the number and function of dopamine neurons. As such, those exposed to cadmium via tobacco smoke or industrial pollution may be at a greater risk for the development of dopamine-related disorders or may display a decreased sensitivity to anti-psychotic medications or drugs used in schizophrenia and Parkinson's," Miller said.

Miller's study recently was published in Pharmacology, Biochemistry and Behavior.